In most applications, the tone mapping process must usually meet two requirements: keep image details, e.g. local contrast and maintain the appearance of relative brightness. Known work on tone mapping focuses on the first requirement and simply neglects the second one, which is usually the most important from the artists' perspective.
High dynamic range (HDR) has received much attention in recent years as an alternative format for digital imaging. The traditional Low Dynamic Range (LDR) image format was designed for displays compliant with ITU-R Recommendation BT 709 (a.k.a. Rec. 709), where only two orders of magnitude of dynamic range can be achieved. Real world scenes, however, have a much higher dynamic range, around ten orders of magnitude in daytime, and the human visual system (HVS) is capable of perceiving 5 orders of magnitude at the same time. Traditional photography has used region-based exposure adjustment for many years, and it continues being used today with digital images. The process, however, is manual and very time consuming.
There are many tone mapping algorithms available in the literature. The best methods (local tone mapping methods) try to adapt the parameters of the tone mapping to local characteristics of the image. Some methods determine the mapping parameters based on the characteristics of a neighborhood around each pixel (e.g. F. Drago, K. Myszkowski, T. Armen, and N. Chiba. “Adaptive Logarithmic Mapping for Displaying High Contrast Scenes,” Computer Graphics Forum, 22(3), 2003; Reinhard, E., Stark, M., Shirley, P., and Ferwerda J. “Photographic tone reproduction for digital images”, ACM Trans. Graph. 21, 3 (July), 267-276; F. Durand and J. Dorsey. “Fast Bilateral Filtering for the Display of High-dynamic-range Images,” ACM Transactions on Graphics, 21(3):257-266, 2002), and others operate on a transformed domain (e.g. R. Fattal, D. Lischinski, and M. Werman. “Gradient Domain High Dynamic Range Compression,” ACM Transactions on Graphics, 21(3):249-256, 2002), which operates on the image gradient), but since none of these methods process the image in regions, they do not allow independently controlling the exposure of different areas of the image.
One method process the image in regions, using several LDR images of the same scene (each generated with a different exposure) as input to the tone mapping process. However, it does not implement any sort of exposure correction.
Tone mapping is not only studied by image processing researchers, but also by painters as well as film photographers. They face the same problem of using a limited dynamic range media (i.e. canvas for painters and print paper for photographers) to represent the high dynamic range scenes. Here we review the “Zone System”, which is a photographic technique formulated by Ansel Adams and Fred Archer (Adams, A. “The Negative”, The Ansel Adams Photography series. Little, Brown and Company. 1981; Adams, A. “The Print”, The Ansel Adams Photography series. Little, Brown and Company. 1981). As shown in FIG. 1, the Zone System assigns numbers from 0 through 10 to different perceptual brightness, with 0 representing black, 5 middle gray, and 10 pure white. These values are known as zones. In the theory of the Zone System, a photographer first identifies the key elements in the scene and places these elements on the desired zones.
This process relies on the perception of the scene rather than on the measurement of the radiance. Then a light meter is used to measure the radiance for each key element in the scene. As there can be only a single exposure value per shot, an exposure value is chosen such that the most important element is mapped to the desired zone. As a result, other (also important) elements may be mapped to the “wrong” zone, becoming either too dark or too bright. Afterwards, in the printing process, this problem is fixed by applying a “dodge and burn” operation, which is a printing technique where some light is withheld from a portion of the print during development (dodge), or more light is added to that region (burn). Therefore, a key element that is mapped to a lower zone than the desired one will be exposed in the light longer than the rest part of the picture. Similarly, the key element that is mapped to a higher zone than the desired one will be exposed less. This local processing will guarantee that the key elements of the picture are mapped to the desired zone in the final output. In other words, the perceptual brightness of these key elements remains consistent with how they look like in real life.